Monday, 10 November 2014

Happy as a Hog in Chalk

Today we've got a blog by the BGS Geology & Landscapes scientific team who are building a digital 'physical properties' model of the Chalk - one of the UK’s most important geological formations.

A combination of 3D modelling and targeted survey work is helping build a detailed understanding of the Chalk’s engineering and hydrogeological variations, and answering fundamental research questions about its geological development. The team consist of Mark Woods, Andy Farrant, Keith Westhead, Andy Newell, Mike Raines, Richard Haslem and Helen Smith and here's more from them...

The Hog’s Back Chalk escarpment near Guildford

The people of Farnham and Guildford in Surrey will be familiar with the great geological feature that links their towns – the Hog’s Back. This high rolling Chalk ridge lies at a geological ‘pinch’ between the wider downlands to the west and the spine of the North Downs to the east. It is one of the places in the country (including also the Isle of Wight and Dorset) where the whole Chalk sequence is tilted steeply into what is known as a ‘monoclinal’ fold, causing its outcrop to appear very narrow on the map.

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A snapshot of the 3D Chalk Model for southern and eastern England, created using GOCAD-SKUA by BGS hydrogeologist and modeller Andy Newell. The Chalk forms a key aquifer below London and rises steeply to the surface in a monoclinal fold at the Hog’s Back in Surrey, subject of recent fieldwork by the team.

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The task of the BGS scientific team (led by Mark Woods and Andy Farrant) this September was to investigate the structure and stratigraphy of the ‘upended’ Chalk making up the Hog’s Back, using a combination of digital field surveying, structural geology, geophysics and palaeontology. The aim was to provide the data needed to verify and improve the Chalk 3D Model being developed at the BGS for the whole of southern and eastern England.

Andy Farrant doing his best Gollum impression in the woods – fallen tree roots are a great place to find Chalk fossils!

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The Chalk is nowhere near as uniform as it may first appear. Hidden within it is a complex sequence of harder and softer, whiter and greyer, flinty and not flinty, marly and not marly (meaning ‘muddy’) chalk layers. These layers form the modern Chalk formations– with names such as the Lewes Chalk and Newhaven Chalk – which can be traced across the wider country. In the past, the Chalk was divided into just three units. Nowadays, we can subdivide it into nine different formations, each with distinct engineering and hydrogeological properties. Understanding how thick these layers are, their distribution across the landscape and how they are orientated beneath the ground in three-dimensions is essential for fully understanding the economically important Chalk.﻿

Andy Farrant (L) helps BGS’s Mike Raines (R) set up the ‘Tromino’ passive seismometer for imaging the subsurface structure of the Hog’s Back.

﻿﻿The BGS team used modern digital techniques for the surveying work, including the BGS•SIGMA (System for Integrated Geoscience Mapping) kit. Profiles of the Chalk escarpment were also made by Mike Raines using the innovative ‘Tromino’ passive seismometer, and early results show how it can help picture the subsurface structure of the Chalk as it dips steeply underneath the Hog’s Back.﻿

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Keith using the BGS•SIGMA digital surveying kit to trace the Chalk stratigraphy across country

A preliminary output from the Tromino passive seismometer survey in September shows bedrock layers forming the Hog’s Back plunging steeply to the North (right) as well as possible weathering layers nearer the surface. This innovative technique can rapidly provide information to support twinned surface surveying and 3D modelling (scale in metres) (Image: Mike Raines).

﻿The downland landscape is dotted with many abandoned small Chalk quarries. In the past these pits provided softer chalks to help lime the surrounding fields and flints for building. In Guildford itself, some larger quarries provided harder chalks (clunch) for use as a building material. For the surveying team, these old pits proved an irresistible draw - despite the nettles, brambles and cobwebs! They allow us to see the Chalk close up in old quarry faces, in order to take samples, find crucial fossils and measure any detailed features we can see such as faults and small scale structures giving clues to the formation of the Hog’s Back itself.

An old Chalk quarry face. If you look closely you can see the layers in the Chalk, which in this quarry become more ‘nodular’ higher up. Many of these quarries were investigated during the current survey by BGS palaeontologist Mark Woods.

Samples of Chalk being sorted in the office before being sent off for micro- and macro-fossil analysis. This helps to date the Chalk layers and to constrain the surface mapping and the 3D model linked to it.

Calyx plate from the lower part of the
Newhaven Chalk Formation

You have to have keen eyesight to find some of the key fossils in the Chalk such as the calyx plate (photo right) from the crinoid Marsupites testudinarius; the biostratigraphy can be used as a guide to help map out the different Chalk formations in three-dimensions – this fossil comes from the lower part of the Newhaven Chalk Formation which crops out on the north side of the Hog’s Back.

This fieldwork forms part of the wider work of the ‘Chalk project’ within the Geology & Landscape England Team. The results of this and ongoing field work will used to improve the Chalk 3D model, update the surface geological maps (including DiGMapGB) and to pursue new research directions into one of the most economically important and scientifically interesting geological formations in the UK.

1 comment:

This post prompted a really interesting question on Facebook... "We are extracting a massive amount of water from the chalk for drinking, irrigation, etc. It comes from our taps saturated with hard water minerals - calcium and magnesium, which precipitate out in the kettle, etc. I would like to know how many tonnes per year of chalk is removed from the strata in this way, and whether it could or has led to the formation of sinkholes!" from Sarah J

Here's the comprehensive answer compiled by a whole raft of BGS experts...

The Chalk is a soluble, porous rock that is dissolved through contact with water that is not already saturated with calcium carbonate (the main component of chalk). Rainwater that has passed through the soil is often weakly acidic and most dissolution occurs near the soil-chalk boundary, i.e. in the upper parts of the chalk strata. So it is as water enters the chalk that most dissolution occurs. In general, human abstraction of Chalk groundwater does not increase the amount of recharge that occurs, it just reduces natural discharge volumes, so is not likely to increase the amount of chalk dissolution.

It is possible to make a rough estimate of the amount of chalk dissolved in the water we abstract. The typical hardness of Chalk groundwater is between 200-300 mg per litre. The UK Groundwater Forum estimates that 1250 million cubic metres of Chalk groundwater is abstracted per year. Using these figures, a rough approximation of the amount of Chalk contained in this water is: 1250,000,000,000 litres x 300 mg = 375,000 tonnes per year. The typical intact dry density of Chalk is 1.53 tonnes per cubic metre which equates to 245,000 cubic metres of Chalk! However, this is spread out across the whole Chalk outcrop, resulting in a gradual lowering of the chalk surface, around a few mm per thousand years. This is unlikely to cause sinkhole formation. However, where water flow is concentrated, and particularly if it is relatively acidic, for example where water flows off adjacent impermeable strata such as sand and clay, or flows through a thick clay soil such as the ‘clay-with-flints’, then this can cause enhanced dissolution and the gradual formation of cavities. These can collapse to form sinkholes if triggered by heavy rain or flooding, for example through leaking water mains (e.g. at Fontwell in 1985).